Fluorescent excimers and exciplexes of the purine base derivative 8-phenylethynyl-guanine in DNA hairpins

Enhancement of optical properties of new purine nucleobases containing electron-donating and -withdrawing peripheral groups

Nowadays, a great deal of attention has been focused on synthesizing highly fluorescent unnatural base analogs. This has been motivated by the need to overcome the lack of fluorescence of nucleic acids' natural bases. Fluorescent unnatural base analogs, such as purines, may be used in several applications, such as DNA or RNA optical spectroscopy studies. Moreover, for purines base analogs, the optical properties, for example, emission, can be tunable through molecular engineering, improving their applications as fluorescent probes. Looking in this direction, the synthesis and optical spectroscopic studies of a new set of purines base analogs are of foremost relevance. Here, an increase in the fluorescence quantum yield was observed in molecules with NH-π-CN arrangement. The two-photon absorption (2PA) cross-sections also increased for the lower energy 2PA state. The enhancement of both properties results in a two-photon brightness of 5 and 10 times higher than in compounds lacking the NH-π-CN arrangement. For the higher energy 2PA state, an excited state absorption contribution to the 2PA cross-section values was observed, that was verified through ultrafast transient absorption measurements. The higher 2PA brightness makes the purines base analogs promising candidates as fluorescent probes in RNA and DNA spectroscopic studies.

Mixed Electronic States in Molecular Dimers: Connecting Singlet Fission, Excimer Formation, and Symmetry-Breaking Charge Transfer

ConspectusChromophore aggregates are capable of a wide variety of excited-state dynamics that are potentially of great use in optoelectronic devices based on organic molecules. For example, singlet fission, the process by which a singlet exciton is down converted into two triplet excitons, holds promise for extending the efficiency of solar cells, while other processes, such as excimer formation, are commonly regarded as parasitic pathways or traps. Other processes, such as symmetry-breaking charge transfer, where the excited dimer charge separates into a radical ion pair, can be both a trap and potentially useful in devices, depending on the context. Thus, an understanding of the precise mechanisms of each of these processes is vital to designing tailor-made organic chromophores for molecular optoelectronics.These excited-state phenomena have each been well-studied in recent years and show tantalizing connections as the molecular systems and environments are subtly changed. These seemingly disparate phenomena can be described within the same unifying framework, where each case can be represented as one point in continuum of mixed states. The coherent mixed state is observed experimentally, and it collapses to each of the limiting cases under well-defined conditions. This framework is especially useful in demonstrating the connections between these different states so that we can determine the factors that control their evolution and may ultimately guide the state mixtures to the product state of choice. The emerging picture shows that tuning the electronic coupling through proper arrangement of the chromophores must accompany environmental tuning of the chromophore energies to produce a fully mixed state. Changes in either of these quantities leads to evolution of the admixture and ultimately collapsing the superposition onto a given state, producing one of the photophysical pathways discussed above.In our laboratory, we are utilizing covalent dimers to precisely arrange the chromophores in rigid, well-defined geometries to systematically study the factors that determine the degree of state mixing and its fate. We interrogate these dynamics with transient absorption spectroscopy from the UV continuously into the mid-infrared, along with time-resolved Raman and emission and magnetic resonance spectroscopies to build a complete and detailed molecular level picture of the dynamics of these dimers. The knowledge gained from dimer studies can also be applied to the understanding the dynamics in extended molecular solids. The insight afforded by these studies will help guide the creation of new designer chromophores with control over the fate of the excited state.